1. Field of the Invention
The present invention concerns adjustment, or modification, of the representation of a device's color gamut when that color gamut is expressed in a color appearance space. More particularly, the present invention relates to adjusting a device's color gamut by adjusting the color gamut, in color appearance space, so as to match the appearance of the device's color-neutral axis.
2. Description of the Related Art
A color appearance space defines color coordinates that attempt to describe how colors appear to a viewer, or how such colors are perceived by the viewer. One popular color appearance space is CIECAM97s, whose color coordinates are JCh (cylindrical). These coordinates can also be expressed in rectangular coordinates, called Jab.
One way in which a color appearance space differs from other spaces such as device-independent color spaces (examples of device-independent color spaces include CIEXYZ and CIELAB) is that a color appearance space will attempt to model how a viewer perceives color, taking into account viewing conditions, surround, and background. For example, the same color sample might appear black if seen against a white background or merely dark gray if seen against a black background. A color appearance space would take the difference in background into account, while a device-independent space such as CIEXYZ does not.
Consider a color printer that prints a black color onto a white sheet by a three-color composite process in which cyan, magenta and yellow are simultaneously deposited into the same print area. Current color appearance spaces are flawed in the sense that they would most likely represent the resulting color as being somewhat chromatic. Depending on the hue, current color appearance spaces would indicate that the color was a very dark green or a very dark blue. However, under most viewing conditions and given the white background of the surrounding sheet, most viewers would agree that the resulting color is actually black; accordingly, in an ideal color appearance space the resulting color would register as black exactly.
Moreover, a problem results when measurements of a device's actual output are used to obtain a representation of the device's color gamut in color appearance space. Specifically, actual measurements of the device's output often yield colors in the color appearance space that do not match a viewer's perceptual judgment. Again considering the above example, the actually-measured value of the composite black color, when expressed in color appearance space, is not an exact black value, even though most viewers would perceive the color as black exactly.
As a result of this situation a problem arises during gamut mapping. Specifically, because black colors expressed in color appearance are not black exactly, bizarre color effects arise during gamut mapping since most gamut mapping is performed in color appearance space. If, for example, gamut mapping compresses colors toward the color-neutral axis (as is the case in most gamut mapping techniques), then the aforementioned black color might be mapped out the back end of the color-neutral axis, resulting in a situation where a perceptual black color would reproduce as dark red.